Deflection correction signal generator

ABSTRACT

A deflection waveform correction signal generator comprises a multiplying circuit for generating a pincushion correction signal. A horizontal frequency ramp is generated by a clipped retrace pulse. The ramp generator has an output coupled to an integrating circuit for generating a parabolic shaped signal. The ramp generator and ramp integrator are reset at a horizontal rate by reset pulses of differing duration. The different duration of reset pulses results in the parabolic signal having regions of non-parabolic shape. The parabolic shaped signal is coupled to an input of the multiplying circuit. A control circuit is coupled to the parabolic shaped signal for maintaining a peak amplitude thereof by controllable coupling to the ramp generator. A control loop is coupled to the integrator for controlling an integrator input bias current to generate the parabolic shaped signal at a predetermined time interval.

This invention relates to the field of video display, and in particularto the generation of deflection waveform correction signals for cathoderay tube displays.

BACKGROUND OF THE INVENTION

In a projection type video display, the usual geometrical rasterdistortions associated with a cathode ray tube display may beexacerbated by the use of a curved face plate CRT and the inherentmagnification in the optical projection path. The use of a curved faceplate CRT may offer benefits in a reduction of projection path length,and may also enable optical imaging simplification. However, the tubedeflection may require the generation of specially shaped, highlystable, correction waveforms in order to achieve more stringentconvergence requirements imposed by large screen viewing.

SUMMARY OF THE INVENTION

A pulse generator for deflection waveform correction comprises a sourceof horizontal retrace pulses subject to beam loading effects. A PLL hasa first input coupled to the retrace pulses for sampling at apredetermined voltage amplitude, and a second input coupled to a sourceof separated synchronizing pulses. The PLL controls a frequency of ahorizontal oscillator signal from which the retrace pulses are derived.A horizontal rate pulse generator is coupled to the retrace pulse andgenerates at the predetermined voltage amplitude, a pulse signal fordeflection correction signal generation. The pulse signal and theoscillator output signal have a fixed phase relationship one with theother largely independent of the beam loading effects on the horizontalretrace pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A-E) is a simplified block diagram of a CRT projection displayincluding inventive features and rasters depicting various geometricaldistortions.

FIG. 2 is a simplified schematic drawing showing inventive features ofFIG. 1.

FIG. 3(A and B) depicts various inventive waveforms.

FIG. 4(A and B) depicts the inventive waveforms of FIG. 3 occurringabout the horizontal blanking interval.

FIG. 5 illustrates a concave, curved face plate of a CRT.

DETAILED DESCRIPTION

A video display employing cathode ray tube projection is illustrated inFIG. 1(A). Three cathode ray tubes are mechanically arranged, andoptically coupled, to project images from CRT phosphor display surfacesto form a single raster on a screen. Each CRT displays an essentiallymonochromatic color raster appropriate to the color signal coupledthereto. The color signals are derived from a display signal inputsignal. The center CRT, for example, displaying a green raster, may bepositioned such that the raster center is orthogonal to the screen. Thetwo other tubes are symmetrically displaced from the center tubeposition and consequently only the vertical part of their raster isprojected orthoganally onto the screen. Thus, in the highly simplifiedarrangement of FIG. 1(A), the outer displayed rasters will suffer atrapezoidal geometrical distortion in addition to other geometricaldistortions resulting from electron beam scanning. The cathode ray tubesshown in FIG. 1(A) and FIG. 5, have a curved, concave spherical phosphordisplay surface. FIG. 5 depicts in profile, a front section of a CRThaving a concave spherical display surface, and indicates the radius ofcurvature and display image diagonals for two tube sizes. Curved faceplate cathode ray tubes are manufactured, for example, by MATSUSHITA, astype P16LET07(RJA) red channel, P16LET07(HKA) green channel,P16LET07(BMB) blue channel. Thus, the projected image, composed of thethree rasters in register on the screen, requires corrective deflectionwaveforms to compensate for geometrical distortions resulting from thecombination of electron beam deflection, tube face plate shape andoptical display path.

Various forms of geometrical distortion result from electron beamscanning. For example, FIG. 1(B) illustrates geometrical distortion inthe vertical scanning direction known as North-South pincushiondistortion. With this form of distortion the vertical scanning speed maybe considered to be modulated, producing incorrect positioning, bowingor sagging, of the horizontal line scan structure as shown in FIG. 1(B).A similar distortion of the horizontal line scan structure isillustrated FIG. 1(C) where the line placement is bowed at a multiple ofhorizontal scanning rate. This form of distortion is termed gullwingdistortion. The desired result of deflection waveform correction isillustrated in FIG. 1(D) which represents the displayed, registered,combination of the three colored rasters. In FIG. 1(D) the verticalposition of the horizontal scan lines of each raster have been correctedsuch that they are nominally parallel, one with the other, and anydifferential placement errors have been minimized to largely eliminatethe formation of spurious colored edges or convergence errors. NorthSouth pincushion correction conventionally utilizes a horizontal rateparabola which is modulated with a vertical rate ramp signal. This formof modulated waveshape usually provides adequate correction of verticalpincushion errors. However, the use of concave face cathode ray tubes inconjunction with the projection optics introduces raster edge lineplacement errors which are not fully corrected by the conventionalmodulated parabolic signal. Thus the parabolic waveshape isadvantageously shaped to produce the desired corrective effect at theedge of the raster. In addition the shaped parabolic waveform isadvantageously horizontally phased to compensate for delay, orhorizontal phase shift, resulting from low pass filtering effectsproduced by the combination of slew rate limitation in the auxiliarydeflection amplifier and inductance of the auxiliary deflection coil.

The various methods for reducing displayed geometrical and convergenceerrors are of limited value unless the resultant correction is stablewith temperature variation, and is insensitive to power supply and beamcurrent loading effects.

In FIG. 1(A) video signal is input at terminal A and is coupled to achroma processor 30, which extracts from the signal, color components,for example, red green and blue for display on cathode ray tubes 510,530, 560. The three cathode ray tube displays are optically projected toform a single image on screen 800. The video signal at terminal A isalso coupled to a synchronizing pulse separator 10, which deriveshorizontal HS, and vertical rate sync pulses VS. from the signal. Theseparated horizontal sync pulses HS are coupled to a phase locked loophorizontal oscillator and deflection amplifier 600. Separated verticalsync pulses VS, are coupled to a vertical oscillator and deflectionamplifier 700. The horizontal PLL oscillator and deflection amplifier600 is coupled to three horizontal deflection coils, RH, GH, which areconnected in parallel. Coil RH represents the red horizontal deflectioncoil, and coils GH and GB represent the green and blue horizontaldeflection coils respectively. Similarly, the vertical oscillator anddeflection amplifier 700 is coupled to three vertical deflection coilsconnected in series, where RV represents the red vertical coil, GV andBV the green and blue coils respectively.

Deflection waveform correction is provided by corrective currentscoupled to individual horizontal and vertical auxiliary deflection coilspositioned, for example, on each tube neck. Auxiliary deflection coilsRHC and RVC, deflecting in the horizontal and vertical directionsrespectively, are positioned on the red CRT neck. Similarly, auxiliarydeflection coils GHC and GVC, and BHC and BVC, green and bluerespectively, are located on the green and blue CRT necks. The auxiliarydeflection coils are driven by auxiliary horizontal and verticaldeflection amplifiers 500/505, 520/525, and 540/545 which represent thered, green and blue channels respectively. The red horizontal auxiliarydeflection amplifier 500, comprises a summer/driver amplifier whichdevelops a composite correction signal which is coupled to thehorizontal auxiliary deflection coil RHC. Similarly, for the redvertical auxiliary deflection amplifier 505, and likewise for the greenand blue channels. The composite correction signal is developed bysummation of a selection of signals having particular waveform shapesand individual amplitude control. Horizontal correction signals whichare generated by circuitry within a pulse and waveform generator 20, andare coupled to the red, green and blue horizontal correction summingamplifiers, 500, 520 and 540. An inventive vertical correction signalgenerator 50, shown in greater detail in FIG. 2, generates a correctionsignal which is coupled to the red, green and blue vertical correctionsumming amplifiers, 505, 525 and 545. The vertical correction signalgenerator 50 receives a horizontal retrace signal input HRT, from thehorizontal oscillator and deflection amplifier 600, and a vertical ratesawtooth signal from the pulse and waveform generator 20. The pulse andwaveform generator 20 receives a vertical rate pulse VRT, from thevertical oscillator and amplifier 700 and the horizontal retrace pulseHRT from the horizontal deflection amplifier 600. In addition togenerating deflection drive signals, the pulse and waveform generatorproduces various deflection waveform correction signals with theexception of North/South pincushion correction.

The vertical correction signal generator 50 is shown in detail in FIG.2. The horizontal retrace pulse signal HRT, is used to generate ahorizontal rate ramp signal which is integrated to from a horizontalrate, generally parabolically shaped signal. The parabolic signal isapplied to a modulating circuit which is modulated by a vertical rateramp signal. The modulating circuit generates a modulated signalcomprising the parabolic shaped signal which is amplitude modulatedresponsive to the vertical rate ramp signal. The vertical ramp modulatesthe parabolic signal, linearly reducing the amplitude to zero at thecenter of the field period at which point the polarity is inverted andthe parabolic signal is linearly increased, achieving full amplitude atthe end of the field. The modulated correction signal, depicted in FIG.1(E) at vertical rate, is coupled to the auxiliary deflection amplifiers505, 525, 545, to produce a North South pincushion corrective current inthe auxiliary deflection coils RVC, GVC, BVC, respectively.

A horizontal retrace pulse signal HRT is coupled via a resistor R1 tothe cathode of zener diode clipper D1 which inventively generates aclipped pulse HzC. The horizontal retrace pulse HRT has a nominally 22volt peak amplitude, however the shape and horizontal phasing of thepeak pulse amplitude may be modulated by the video content of thedisplayed image, as illustrated in waveform B of FIGURE 4. Such retracepulse modulation may result in spurious, unwanted horizontal phasemodulation of correction signals relative to the horizontal deflection.The inventive zener diode clipper is selected to have a breakdownvoltage which corresponds to the retrace pulse amplitude value at whichthe horizontal PLL oscillator is synchronized. Since clipped pulse HzC,and the derived correction waveforms, are derived from the same retracepulse amplitude value as the horizontal PLL, unwanted phase modulationbetween deflection and correction signals can be essentially eliminatedensuring that both deflection and correction waveforms track together.The horizontal. PLL is synchronized at a retrace pulse amplitude ofnominally 6.8 volts, hence clipping zener diode D1 is selected to have a6.8 volt break down voltage. Thus the nominally 22 volt retrace pulseHRT containing power supply loading and video dependent amplitude andpulse shape variations are inventively eliminated by the clipping actionof zener diode D1. The zener diode produces a nominal pulse amplitude of7.4 volts peak to peak, which represents +6.8 volts plus -0.7 voltreverse conduction. The inventive use of zener diode clipper D1, largelyremoves video signal and beam current related variations of retracepulse shape and amplitude. Thus undesirable horizontal phase modulationof the correction waveform is largely eliminated. A further advantagearising from zener clipping of retrace pulse HRT prior todifferentiation is the generation of accurate, stable reset pulsewidths, independent of retrace pulse shape, rise time or amplitude.Reset pulses are generated from the same polarity of differentiatedpulse edge. Furthermore the reset pulse may have a duration, or width,greater than half the duration of retrace pulse HRT, which is notpossible if the reset pulse is differentiated without clipping.

The clipped retrace pulse HzC, at the cathode of diode D1 is coupled toa series network comprising a capacitor C1 which is connected to a pairof series connected resistors R2 and R3. Resistor R3 is connected toground and the junction of the resistors is connected to the base of atransistor Q1. The time constant of the series connected network is suchthat the clipped retrace pulse is differentiated and applied to the baseof transistor Q1. The emitter terminal of transistor Q1 is connected toground and the collector terminal is connected to a capacitor C2 via aresistor R4. The emitter terminal of transistor Q2 is connected to a +12volt supply via a resistor R5 and the collector is connected to thejunction of capacitor C2, resistor R4 and the base of a transistor Q3.Transistor Q3 functions as an emitter follower with the collectorterminal connected to ground and the emitter terminal connected to the+12 volt supply via a resistor R6. Transistor Q2 is a constant currentsource where the current magnitude is controlled by signals coupled tothe emitter and base terminals. The collector current of transistor Q2charges capacitor C2 towards +12 volt generating a nominally linear rampof increasing voltage. The differentiated positive edge of the clippedretrace pulse is applied to the base of transistor Q1 causing it tosaturate for approximately 8 microseconds. Thus the ramp voltage, formedacross capacitor C2, is discharged via transistor Q1 and resistor R4.The discharge time constant of ramp forming capacitor C2 is largelydetermined by resistor R4, which is selected to generate anexponentially shaped voltage discharge ramp. The horizontal rate, shapedramp signal is coupled, via emitter follower Q3, to series connectedcapacitor C3 and resistor R7 which are coupled to an inverting input ofan integrating amplifier U1. Amplifier U1 is powered from the +12 voltsupply via a resistor R9, and from the -12 volt supply via a resistorRS. The non-inverting input of amplifier U1 is grounded.

Circuit 100 is an inventive horizontal rate integrator and reset pulsegenerator. The clipped retrace pulse HzC is also coupled to a seriesconnected network comprising a capacitor C100 which is connected to aseries connected pair of resistors R100 and R101. Resistor R101 isconnected to ground and the junction of the resistors is connected tothe base of a transistor Q100. The time constant of the series connectednetwork differentiates the clipped retrace pulse, with the positive edgecausing transistor Q100 to saturate for 5 microseconds generatingintegrator reset pulse IR. The emitter terminal of transistor Q100 isconnected to a resistor R102 which is connected to the output of U1, andthe collector terminal is connected to the inverting input of U1. Thustransistor Q100 discharges or resets, via resistor R 102, theintegrating capacitor. C2 of the integrator formed by I.C. UI. Since thedischarge time constant of resistor R102 and integrating capacitor C101is short, approximately 0.5 microseconds, integrator capacitor C101 israpidly discharged and held reset for the remainder of the conductionperiod.

The ramp signal is coupled via capacitor C3 and resistor R7, to theinverting input of amplifier U1. The output terminal of amplifier UI iscoupled, via integrating capacitor C101, back to the inverting inputthus causing the ramp signal be integrated, and generating a generallyparabolic shaped output signal P. The output signal P, of integrator U1is connected to a clipper or active clamp advantageously formed byadvantageous circuit 200. Parabolic correction signal P is connected toan emitter terminal of a transistor Q200. The collector of transistorQ200 is connected to ground and the base is connected to the base of atransistor Q201. The base and collector terminals of a transistor Q201are connected together and the emitter is connected to ground. Thustransistor Q201 functions as a forward biased voltage reference diodewhich accurately determines the Vbe of clipper transistor Q200. Thejunction of the base and collector terminals of transistor Q201 arecoupled to the +12 volt supply via a resistor R200, which limits thecollector current to approximately 1 milliamp. The current gain oftransistor Q201, for example 100, establishes a base current of about 10microamps. The connection of transistor Q201 base and collectorterminals results in feedback which generates a base/collector toemitter potential of approximately 0.5 volts, set by the base current of10 microamps. The 0.5 volt developed across transistor Q201 is appliedto the base of transistor Q200 and thus establishes a temperature stableclamping potential at the transistor Q200 emitter.

The output terminal of amplifier U1, for example IC type TLO82, isconnected to the emitter of transistor Q200. Amplifier U1 has aninternal current limitation of approximately ±25 milliamps, hence thisdetermines the maximum current which may be conducted by transistor Q200during clamping. Transistor Q200 has a current gain of, for example 100,thus, during clamping a base current of approximately 250 microampsresults, with a Vbe of approximately 0.6 volts. Since the base toemitter voltages of transistors Q200 and Q201 are tied together andtrack with temperature, a clamping potential of approximately -10millivolts is established at transistor Q200 emitter. Thus negativesignal excursions at the output terminal of integrator U1 are limited bythe clamping action of transistor Q200 emitter to approximately -100millivolts.

The parabolic shaped signal output P, of integrator U1 is also connectedvia series connected resistor R10 and capacitor C4 to the emitter oftransistor Q2 to advantageously provide modulation of the ramp formingcurrent generated at the collector. The parabolic shaped modulationcurrent, injected at the emitter of transistor Q2 causes the ramp slopeto be reduced at the beginning and end. Thus when the ramp is integratedby integrator U1, a modified parabolic shape results which is requiredfor gullwing distortion correction. It was discovered that, unlikeprevious sources of gullwing deflection distortion, the particulargullwing errors resulting from the use of curved or concave face platetubes required gullwing correction having an opposite polarity to thatpreviously employed.

The parabolic shaped output signal P, from integrator U1 is also coupledto an advantageous amplitude control circuit 300. Control circuit 300compares the amplitude of parabola P against a zener diode derivedreference voltage and generates an output voltage which is applied tothe ramp current source generator to form a negative feedback controlloop. Thus advantageous amplitude control circuit 300 provides a controlloop which may correct for amplitude variations resulting duringhorizontal ramp and parabolic signal generation. The parabolic shapedsignal P is coupled to the base of a transistor Q300 which is arrangedwith a transistor Q301 as an emitter coupled or differential amplifier.The emitter terminal of transistor Q300 is connected to ground via aparallel combination of a resistor R300 and a capacitor C300. Theemitter of transistor Q300 is also connected to an emitter terminal oftransistor Q301 via a resistor R301 which provides gain degeneration andaids control loop stabilization. The base terminal of transistor Q301 isconnected to a DC reference potential generated at the junction of azener diode D300 and a resistor R303. Resistor R303 is connected betweenthe +12 volt supply and the cathode of a zener diode D300, the anode ofwhich is connected to ground. Zener diode D300 has a breakdown voltageof 5.6 volts which is applied to the base of transistor Q301 and resultsin approximately 5 volts appearing across capacitor C300. It isdesirable that signal P be generated with a maximal amplitude. However,too larger parabolic signal amplitude may cause transistor Q100 tobreakdown and clip signal P. Thus a maximum amplitude of 5.6 volts isselected to avoid break over effects in transistor Q100. The collectorof transistor Q301 is connected to a parallel combination of a resistorR302 and a capacitor C301 which are connected to the +12 volt supply.Resistor R302 and capacitor C301 form a low pass filter which smoothesthe horizontal rate current pulses and generates a control voltage whichis coupled to the base of transistor Q2 to control the amplitude of themodulated current source. The parabolic shaped signal P, coupled to thebase of transistor Q300 causes current flow when the parabolic waveformpeak exceeds the voltage across capacitor C300 plus the Vbe potential oftransistor Q300. Thus parabolic waveform peaks greater than nominally5.6 volts cause the voltage across capacitor C300 to increase. Theincreased voltage across capacitor C300 results in a decrease in the Vbepotential of transistor Q301 which reduces the collector current flow.Hence the voltage developed or dropped, across resistor R302 andcapacitor C301 and coupled to transistor Q2 base, is reduced. Thus, thecurrent in ramp forming transistor Q2 is controllably reduced, reducingthe ramp amplitude and restoring the amplitude of parabolic signal P to5.6 volts. Inventive amplitude control loop 300 includes the ramp andparabola generators and maintains the peak amplitude of the parabolicshaped signal P equal to the voltage across diode D300. Thus theamplitude of correction signal P is maintained essentially constant andindependent of power supply and component variations.

An advantageous pulse width control circuit 400, generates a directcurrent which is coupled to the inverting input U1. Integration of thisDC by I.C. U1 results in a horizontal rate, tilt or ramp component beingadded to the horizontal rate parabolic signal P. The inverting input ofintegrating amplifier U1 is connected via a resistor R409 to anadvantageous pulse width control circuit 400. The direct current coupledvia resistor R409 is derived from measurement of a pulse width withreference to a divided potential derived from the positive and negative12 volt power supplies. As described for advantageous circuit 200,negative excursions of the parabolic signal P are clamped to -100millivolts by circuit 200. The clamping action circuit 200 sinks currentfrom the output circuitry of amplifier U1, resulting in currentlimitation due to the current limiter within I.C. U1. The outputcircuitry of I.C. U1 remains in the current limited condition for theduration of the clamped negative signal excursion. The current limitingcondition within amplifier U1 may be observed by monitoring the currentsourced by the -12 volt supply. For example, at the onset of clippingthe current will increase to the limiting value and remain there for theduration of clipping. Since the - 12 volt supply is coupled via aresistor R8, the supply current step to the limitation value will resultin a voltage step or pulse, due to the voltage drop across supplyresistor R8. Thus current limitation in I.C. U I generates a positivepulse PC, at the junction of resistor R8 and I.C. U1, having a durationequal to the duration of the clamping action of circuit 200. Pulse PC,is coupled to series connected resistors R401 and R402. Resistor R402 isconnected to the -12 volt supply and the junction of the resistors formsa potential divider which is connected to the base terminal of atransistor Q400. Transistor Q400 functions as a saturating switch, withthe emitter terminal connected to the -12 volt supply. The collectorterminal of transistor Q400 is connected via a resistor R404 to the +12volt supply. Transistor Q400 collector is also connected to a low passfilter formed by series connected resistor R403 and shunt connectedcapacitor C400. Capacitor C400 is connected to the +12 volt supply withthe junction connected to the base terminal of an emitter coupledamplifier transistor Q401. The collector terminal of transistor Q401 isconnected to ground and the emitter is connected to the +12 volt supplyvia a resistor R405. The emitter of Q401 is also coupled to the emitterterminal of a transistor Q402 via a resistor R406. Transistors Q401 and402 may be considered as a differential amplifier with gaindegeneration, or loop damping, resulting from resistor R406 intransistor Q402 emitter. The base of transistor Q402 is connected to thejunction of resistors R407 and R408 which form a potential dividercoupled between the positive and negative 12 volt supplies. ResistorR408 is connected to the -12 volt supply and resistor R407 is connectedto the +12 volt supply. The collector terminal of transistor Q402 isdecoupled to ground by a capacitor C401 and is connected to theinverting input of integrating amplifier U1 via a resistor R409.

The positive pulse PC, at resistor R8, is amplified and inverted bytransistor Q400. The inverted collector pulse is low pass filtered, orintegrated, by resistor R403 and capacitor C400 to produce a DC voltageVPC. The low passed DC voltage VPC, has an amplitude which varies inproportion to the width of pulse PC. Voltage VPC is coupled thedifferential amplifier formed by transistors Q401 and Q402 where it iscompared with a reference DC voltage generated by potential dividerresistors R407 and R408. The potential divider is coupled between thesupply voltages which power the integrator and allied circuitry, thusvariations in either supply will result in a change to the referencepotential and a compensating correction in pulse width. To improve theaccuracy of pulse PC, resistors R407 and R408 have 2% resistance valuetolerances. The potential divider generates a reference voltage equal toa ratio of 11/63.5 of the voltage existing between the positive andnegative 12 volt supplies. The ratio of 11/63.5 represents the width, orduration, of pulse PC as a ratio of the horizontal period. Thus,variations of voltage VPC are compared with the reference voltage, whichrepresents the desired pulse duration, and cause a corrective current toflow in transistor Q402. The corrective current IT, is coupled viaresistor R409 to vary the bias current at the inverting input ofamplifier U1. The integrated effect of the corrective DC bias currentIT, introduced by resistor R408, is to cause the output signal of U1 tobe superimposed on a shallow ramp having a slope proportional to thecurrent IT. Thus parabolic signal P is tilted, causing the waveformcusps to have different DC potentials, with the result that negativeparabolic signal excursions are clamped by circuit 200. The clampingaction results in the generation of current limit pulse PC, having awidth or duration which is controlled responsive to the corrective biascurrent IT. The advantageous pulse width control circuit 400, forms acontrol loop which compensates for variations in, power supplies and theclamping voltage of circuit 200.

The shaped parabolic correction signal P, from integrator U1, is coupledto a balanced modulator I.C. U2 which generates a vertical ratepincushion correction signal. The modulated output signal from I.C. U2is coupled via correction amplitude controls to auxiliary deflectionamplifiers, 505, 525 and 545 and auxiliary vertical deflection coilsRVC, GVC and BVC which correspond respectively to the red, green andblue colour projection tubes. Integrated circuit U2 generates suppressedcarrier amplitude modulation of the horizontal rate parabolic signalwith a vertical rate sawtooth signal to produce the modulated waveform,or bow tie signal, illustrated in waveform E of FIG. 1.

FIG. 3(A) illustrates various waveforms and their timing relationshipsdepicted during a horizontal interval and time referenced to the startof the horizontal retrace pulse HRT. The signal amplitudes in (A) arefor illustrative purposes only. Retrace pulse HRT may be derived, forexample, from a CRT heater winding on a horizontal deflection outputtransformer, and may have a pulse amplitude of approximately 22 volts.The pulse depicted in (A) has a nominal duration of approximately 12microseconds and is illustrated without typical shape, width and risetime modulations resulting from various loading mechanisms. Waveform Rrepresents the horizontal rate ramp R occurring at the collector oftransistor Q2 of FIG. 2. Ramp R is depicted with a linear ramp up sincegullwing corrective shaping is omitted for drawing clarity. However, theexponential reset period resulting from the action of discharge resistorR4 is shown. The shaped parabolic signal is depicted by waveform P,generated at the output of I.C. U1 in FIG. 2. The specific parabolicsignal start and stop times are more accurately depicted in waveform (A)of FIG. 4. However, the advanced horizontal phase of parabolic signal Prelative to horizontal retrace pulse HRT, illustrates the advantageousphase advance required to compensate for delaying effects present in thecorrection signal path. Hence deflection correction is provided which ishorizontally centered with respect to horizontal deflection.

Waveform (B) of FIG. 3, illustrates the modulated parabolic correctioncurrent IGOR, for example, in the green vertical correction coil GVC.Correction current IGOR is depicted centered in the horizontal rasterscan. When waveform ICOR is viewed at horizontal rate, as in (B), allvertical scan lines are superimposed, hence the vertical rate modulationapplied to the parabolic signal effectively fills in the waveform asdepicted. Furthermore, waveform ICOR is depicted with two apparentparabolic signals of differing amplitudes, this depiction results fromthe use of suppressed carrier amplitude modulation of signal P by thevertical sawtooth. Shaped parabolic signal P is illustrated with a phaseadvance A, for example, 5 microseconds, required to horizontally centerthe corrective effect produced by the auxiliary deflection/correctioncoil. The shaped parabolic correction signal is advanced in time as aresult of the tilt component generated by the bias current introducedvia resistor R409.

In FIG. 4, waveform (A) depicts the horizontal phasing of variouswaveforms utilized to generate the inventive corrective waveform shape.Correction waveform P, although nominally parabolically shaped,comprises various additional waveshapes which provide specificcorrection at specific raster locations. Waveform (A) shows the phasingof the horizontal retrace pulse HRT at time t0, relative to variouswaveforms occurring during the display of left and right raster sides.At time RHS, t3-t0, the right hand side of the raster is displayed, andcorrection waveform P is shaped by the clamping action circuit 200.Inventive clamp 200 clips the negative cusps or peaks which results inzero corrective waveform amplitude during time t3-t0, for example, 5microseconds. The saturation, or current limit pulse PC is depictedoccurring at time RHS, t3-t0. The falling edge of pulse PC is coincidentwith the start of the integrator reset pulse IR, since pulse IR ends theintegration period and thus ends parabola generation. Any instability inhorizontal timing of waveform P will be indicated by movement of theleading edge of pulse PC, which changes the pulse width. Althoughwaveform P is reduced to zero at time RHS, during the right side of theraster is displayed, the actual modulated current ICOR, in therespective correction coils is not only delayed but also suffers adegradation in rise/fall times. Thus the apparent abrupt waveformdiscontinuity of signal P at time t3 is smoothed or flared towards azero correction value. The horizontal phasing, or starting point t1, ofcorrection waveform generation is determined by the integrator resetpulse IR. When pulse IR ends at time t1, capacitor C101 is allowed tointegrate and initiate generation of correction waveform signal P. Attime LHS, t1-t2, the left hand side of the raster is displayed, andcorrective waveform P is shaped by integration of the exponential shapeEXP, occurring during times t1-t2. The exponential shape is generated bythe discharge of capacitor C2 via resistor R4. During time LHS,correction waveform P has a shape resulting from integration of theexponentially shaped discharge portion of ramp signal R. At time t2,ramp reset pulse RR ends, exponential discharge ceases, and linear rampgeneration is initiated. Thus for trace time, between t2-t3, ramp R isintegrated producing the parabolic shaped component of correctionwaveform P. After time t2, linear ramp R is generated with gull wingcompensation, which following integration, produces the nominallyparabolically shaped correction signal P.

The parabolic signal component of correction signal P provides northsouth pincushion correction when modulated by the vertical sawtooth. Inaddition to pincushion correction, the parabolic signal component isshaped to provide gull wing correction, and has further shaping orflaring to provide left and right raster edge correction. Hence theparabolically shaped correction signal, is generated with an advancedhorizontal phase, comprising various regions, shaped to producecorrective effects at specific raster locations. Thus accurately definedand stably implemented deflection correction is provided for highquality image display.

What is claimed is:
 1. A pulse generator for deflection waveformcorrection comprising:a source of horizontal retrace pulses subject tobeam loading effects; a PLL having a first input coupled to said retracepulses for sampling at a predetermined voltage amplitude, a second inputcoupled to a source of separated synchronizing pulses, said PLLcontrolling a frequency of a horizontal oscillator signal for derivingsaid retrace pulses therefrom; and, a horizontal rate pulse generatorcoupled to said retrace pulses and generating at said predeterminedvoltage amplitude, a pulse signal for deflection correction signalgeneration, said pulse signal and said oscillator output signal having afixed phase relationship one with the other largely independent of saidbeam loading effects on said horizontal retrace pulses.
 2. The pulsegenerator of claim 2, wherein said horizontal rate pulse generatorfurther comprises a zener diode clipper coupled to said horizontalretrace pulse and generating therefrom said pulse signal, said zenerdiode having a breakdown voltage for clipping at said predeterminedvoltage amplitude.
 3. The pulse generator of claim 1, wherein saidhorizontal rate pulse generator further comprises a first reset pulsegenerator, triggered by a first trigger signal derived bydifferentiation of said pulse signal.
 4. The pulse generator of claim 3,wherein said horizontal rate pulse generator further comprises a secondreset pulse generator, triggered by a second trigger signal derived bydifferentiation of said pulse signal.
 5. The pulse generator of claim 4,wherein said first and said second trigger signals being derived from acommon edge of said pulse signal.
 6. The pulse generator of claim 4,wherein a first reset pulse and a second reset pulse startcoincidentally.
 7. The pulse generator of claim 6, wherein one of saidfirst and second reset pulses having a duration less than nominally halfa duration of said horizontal retrace pulse, and said other one of saidfirst and second reset pulses having a duration greater than nominalhalf said duration of said horizontal retrace pulse.
 8. A deflectioncorrection waveform generator comprising:a source of horizontal retracepulses clipped at a predetermined amplitude; a first reset pulsegenerator coupled to said clipped horizontal retrace pulses andtriggered from an edge thereof, said first reset pulse generatorgenerating a first reset pulse of duration greater than half saidhorizontal retrace pulse duration; a second reset pulse generatorcoupled to said clipped horizontal retrace pulses and triggered fromsaid edge, said second reset pulse generator generating a second resetpulse of duration less than half said horizontal retrace pulse duration;a horizontal rate ramp signal generator, reset by said first reset pulsegenerator; and, an integrator coupled to said ramp generator and resetby said second reset generator, said integrator generating a generallyparabolically shaped signal therefrom.
 9. The deflection correctionwaveform generator of claim 8, wherein said horizontal rate rampgenerator, generates a ramp signal being essentially linear during atrace interval and having an exponential shape during during a retraceinterval.
 10. The deflection correction waveform generator of claim 9,wherein said exponential shape being generated by a discharge switchresponsive to said first reset pulse.
 11. The deflection correctionwaveform generator of claim 9, wherein said discharge switch generatessaid exponential shape for a time period greater than nominally half aduration of said horizontal retrace pulse.
 12. The deflection correctionwaveform generator of claim 9, wherein said generally parabolicallyshaped signal comprises a first interval having a first shape determinedby integration of said exponential shaped ramp and a second intervalhaving a second shape determined by integration of said essentiallylinear ramp.